Crustose Coralline Algae Species Research Articles

The State of Knowledge of CCA Diversity in the Caribbean Coral Reefs

Crustose coralline algae (CCA) are a diverse and ecologically important species found in most of the world’s oceans. The current lack of taxonomic knowledge and relative abundance compromises our ability to predict species diversity numbers and, thus, their ecological roles and impacts on coral reefs. To gather a better understanding of the state of knowledge of crustose coralline algae taxonomy in the Caribbean, 107 different research papers, and other primary and secondary literature were studied; any source with taxonomical information, species identification, or genetic markers for identification was recorded. All Genebank codes were collected and sorted by supposed species marker and then ran through the National Center of Biotechnology Information. The location these genetic markers were gathered from was compared to the natural habitat range of the species, based on the Algaebank habitat description. Of the supposed 83 described species of crustose algae in the Caribbean, based on morphological characteristics, only 24 total were confirmed by DNA markers. This leaves at least 59 species of CCA to be confirmed in the Caribbean Sea with molecular markers. This indicates the importance of DNA barcode survey studies to assess the accurate diversity of this group in the region. With this limited knowledge apparent, it should be seen that a CCAs phylogenetic and taxonomical review must be done. An in-depth assessment should be conducted on CCA collections to identify the Caribbean species correctly and thus know their biodiversity in local habitats.

Seasonal growth and calcification of three species of crustose coralline algae in Moore, French Polynesia

Seasonal growth and calcification of three species of crustose coralline algae in Moore, French Polynesia

Investigating seed bank potential of crustose coralline algae using DNA metabarcoding.

To examine the potential for the autogenic ecosystem engineers, crustose coralline algae (CCA), to serve as seed banks or refugia for life stages of other species, it is critical to develop sampling protocols that reflect the diversity of life present. In this pilot study on two shallow water species of CCA collected from Raoul Island (Kermadec Islands; Rangitāhua) New Zealand, we investigated two preservation methods (ethanol vs. silica gel), sampled inner and outer regions of the crusts, and used DNA metabarcoding and seven genes/gene regions (16S rRNA, 18S rRNA, 23S rRNA, cox1, rbcL, and tufA genes and the ITS rRNA region) to develop a protocol for taxa identification. The results revealed immense diversity, with typically more taxa identified within the inner layers than the outer layers. As highlighted in other metabarcoding studies and in earlier work on rhodoliths (nodose coralline algae), reference databases are incomplete, and to some extent, the use of multiple markers mitigates this issue. Specifically, the 23S rRNA and rbcL genes are currently more suitable for identifying algae, while the cox1 gene fares better at capturing the diversity present inclusive of algae. Further investigation of these autogenic ecosystem engineers that likely act as marine seed banks is needed.

Hierarchical settlement behaviours of coral larvae to common coralline algae

Natural regeneration of degraded reefs relies on the recruitment of larvae to restore populations. Intervention strategies are being developed to enhance this process through aquaculture production of coral larvae and their deployment as spat. Larval settlement relies on cues associated with crustose coralline algae (CCA) that are known to induce attachment and metamorphosis. To understand processes underpinning recruitment, we tested larval settlement responses of 15 coral species, to 15 species of CCA from the Great Barrier Reef (GBR). CCA in the family Lithophyllaceae were overall the best inducer across most coral species, with Titanoderma cf. tessellatum being the most effective species that induced at least 50% settlement in 14 of the coral species (mean 81%). Taxonomic level associations were found, with species of Porolithon inducing high settlement in the genus Acropora; while a previously understudied CCA, Sporolithon sp., was a strong inducer for the Lobophyllidae. Habitat-specific associations were detected, with CCA collected from similar light environment as the coral inducing higher levels of settlement. This study revealed the intimate relationships between coral larvae and CCA and provides optimal coral-algal species pairings that could be utilized to increase the success of larval settlement to generate healthy spat for reef restoration.

Cell wall organic matrix composition and biomineralization across reef-building coralline algae under global change.

Crustose coralline algae (CCA) are one of the most important benthic substrate consolidators on coral reefs through their ability to deposit calcium carbonate on an organic matrix in their cell walls. Discrete polysaccharides have been recognized for their role in biomineralization, yet little is known about the carbohydrate composition of organic matrices across CCA taxa and whether they have the capacity to modulate their organic matrix constituents amidst environmental change, particularly the threats of ocean acidification (OA) and warming. We simulated elevated pCO2 and temperature (IPCC RCP 8.5) and subjected four mid-shelf Great Barrier Reef species of CCA to 2months of experimentation. To assess the variability in surficial monosaccharide composition and biomineralization across species and treatments, we determined the monosaccharide composition of the polysaccharides present in the cell walls of surficial algal tissue and quantified calcification. Our results revealed dissimilarity among species' monosaccharide constituents, which suggests that organic matrices are composed of different polysaccharides across CCA taxa. We also observed that species differentially modulate composition in response to ocean acidification and warming. Our findings suggest that both variability in composition and ability to modulate monosaccharide abundance may play a crucial role in surficial biomineralization dynamics under the stress of OA and global warming.

Geographic distance, sedimentation, and substrate shape cryptic crustose coralline algal assemblages in the world's largest subtropical intertidal algal reef.

Algal reefs, concreted by crustose coralline algae (CCA), are the main biotic reefs in temperate waters but rare in the subtropics and tropics. The world's largest known intertidal algal reef in the subtropics is the Taoyuan Algal Reef (TAR) located in the northwestern coast of Taiwan. The biodiversity and ecology of the TAR are scarcely explored, and now the reef is imperiled by industrialization. Here, we document cryptic species of CCA in Taiwan, particularly the TAR, by sequencing the psbA genes of over 1800 specimens collected across Taiwan. We also examine the ecological background of the TAR by surveying its benthic composition and measuring its environmental parameters. Our data reveal that the TAR harbours a high diversity of cryptic CCA species (27 molecular operational taxonomic units, or mOTUs), many of which are potentially new to science (18 mOTUs) and/or endemic to the TAR (9 mOTUs). Comparing the CCA species inventory of the TAR with the rest of Taiwan shows that the TAR represents a unique hotspot of CCA taxa in the waters of Taiwan. Our analyses show that variation in the CCA assemblages in the TAR is associated with geographic distance, sedimentation, and substrate type (for example, reef vs. hermit crab shell), suggesting that dispersal limitation and contemporary environmental selection shape the CCA assemblages in the TAR. The data from this study can inform the monitoring of human impacts on the health of the TAR and contribute to our understanding of the ecological processes underlying algal reef development.

Coral larval settlement and post-settlement survival facilitated by crustose coralline algae with or without living tissue

Selection of a permanent attachment site of coral larvae can be a critical determinant of recruitment success affecting the structure of coral communities and underpins the ability of coral reef ecosystems to recover from disturbance. Settlement specificity of a threatened coral in Sanya reefs, Acropora millepora, was tested by measuring the larval metamorphosis preferences and post-settlement survival in response to crustose coralline algae (CCA) species Hydrolithon reinboldii and other substrata. In the no-choice experiments, the larvae of A. millepora had similar rates of total metamorphosis with the presence of CCA regardless of the algae tissue being alive or not, and settlement success induced by CCA was higher than by other substrata (tile or glass). In the paired-choice experiments, when CCA was in presence, the coral larvae preferred the surface of the dish and the side of living CCA. In the absence of CCA, total larvae metamorphosis was lower than in the treatments where CCA was present. New recruits of A. millepora had approximately 68% mean survival on all the settlement substrata after 2 weeks maintained in aquaria with flow-through seawater similar to the coral larval sampling site, but with no coral spat survival in the treatments where CCA was absent. However, there were statistical differences between the larvae survival of dead CCA and glass chips treatment and the others where CCA was present. Our results were consistent with the conclusion that some CCA species could facilitate coral larval settlement and post-settlement survivorship, highlighting the importance of substrata selection success for facilitating coral recruitment in the threatened coral reefs.

Coral larval settlement preferences linked to crustose coralline algae with distinct chemical and microbial signatures

The resilience of coral reefs is dependent on the ability of corals to settle after disturbances. While crustose coralline algae (CCA) are considered important substrates for coral settlement, it remains unclear whether coral larvae respond to CCA metabolites and microbial cues when selecting sites for attachment and metamorphosis. This study tested the settlement preferences of an abundant coral species (Acropora cytherea) against six different CCA species from three habitats (exposed, subcryptic and cryptic), and compared these preferences with the metabolome and microbiome characterizing the CCA. While all CCA species induced settlement, only one species (Titanoderma prototypum) significantly promoted settlement on the CCA surface, rather than on nearby dead coral or plastic surfaces. This species had a very distinct bacterial community and metabolomic fingerprint. Furthermore, coral settlement rates and the CCA microbiome and metabolome were specific to the CCA preferred habitat, suggesting that microbes and/or chemicals serve as environmental indicators for coral larvae. Several amplicon sequence variants and two lipid classes—glycoglycerolipids and betaine lipids—present in T. prototypum were identified as potential omic cues influencing coral settlement. These results support that the distinct microbiome and metabolome of T. prototypum may promote the settlement and attachment of coral larvae.

Acclimation History of Elevated Temperature Reduces the Tolerance of Coralline Algae to Additional Acute Thermal Stress

Increasing atmospheric CO2 is driving major environmental changes in the ocean, such as an increase in average ocean temperature, a decrease in average ocean pH (ocean acidification or OA), and an increase in the number and severity of extreme climatic events (e.g., anomalous temperature events and heatwaves). Uncertainty exists in the capacity for species to withstand these stressors occurring concomitantly. Here, we tested whether an acclimation history of ocean warming (OW) and OA affects the physiological responses of an abundant, reef-building species of crustose coralline algae (CCA), Porolithon cf. onkodes, to chronic and acute thermal stress. To address this, we exposed algae to varying temperature and pH levels for 6 weeks and this chronic treatment experiment was followed by an acute exposure to an anomalous temperature event (+4–6°C from acclimation temperature). Net photosynthetic rate was negatively affected across all treatments by increasing temperature during the acute temperature event, however, algae acclimated to the control temperature were able to maintain photosynthetic rates for +4°C above their acclimation temperature, whereas algae acclimated to elevated temperature were not. Average relative change in O2 produced resulted in a 100–175% decrease, with the largest decrease found in algae acclimated to the combined treatment of elevated temperature and reduced pH. We conclude that acclimation to chronic global change stressors (i.e., OW and OA) will reduce the tolerance of P. cf. onkodes to anomalous increases in temperature, and this may have implications for reef building processes.

Bioerosion of reef-building crustose coralline algae by endolithic invertebrates in an upwelling-influenced reef

Coral reef growth is primarily determined by constructive and bioerosive processes acting on key reef-building organisms. Among them, corals are major contributors to the construction of reef frameworks, while crustose coralline algae (CCA) primarily take part in reef cementation. Despite the significance of CCA for reef ecology and functioning, there is very little information on CCA bioerosion rates, in particular in reefs influenced by coastal upwelling. Therefore, the aim of this study was to examine the percentage and rates of internal bioerosion by macroborer invertebrates in two dominant CCA species and to explore whether the oceanographic variability influences the nature of bioerosion in coral reefs of the Tayrona Natural National Park (Colombian Caribbean). Annual rates of gross calcium carbonate production for Lithoplyllum kaiseri and Porolithon antillarum were 0.556 (± 0.38) and 0.883 (± 1.0) g CaCO3 cm−2 year−1, respectively, and estimates of percent area covered by macroborer boreholes showed values of 29.7% for L. kaiseri and 18.0% for P. antillarum. Rates of calcium carbonate removal by internal macroborers in L. kaiseri (0.19 ± 0.17 g CaCO3 cm−2 year−1) were higher than those in P. antillarum (0.14 ± 0.17 g CaCO3 cm−2 year−1). The percentage of internal bioerosion did not vary significantly across climatic/oceanographic seasons. Boreholes produced by mollusks, polychaetes, sponges and sipunculids were identified, with mollusks having the highest erosion activity. A total of 2,095 individuals of boring and opportunistic organisms were identified and grouped into 29 families (17 polychaetes, 4 mollusks, 2 sipunculids and 6 crustaceans). The composition of the macroborer invertebrate community also varied between CCA species, with higher boreholes attributed to vermetids in L. kaiseri than in P. antillarum. Although there is no clear influence of climatic seasons on internal bioerosion, the high rates of CCA bioerosion may reduce reef consolidation in the region.

An unusual microbiome characterises a spatially-aggressive crustose alga rapidly overgrowing shallow Caribbean reefs

Several species of crustose coralline algae (CCA) and their associated microbial biofilms play important roles in determining the settlement location of scleractinian corals on tropical reefs. In recent decades, peyssonnelid algal crusts (PAC) have become spatial dominants across large areas of shallow Caribbean reefs, where they appear to deter the recruitment of scleractinians. Our genetic investigations of PAC in St. John, US Virgin Islands, amplifying the large-subunit ribosomal RNA and psbA protein D1 marker genes, revealed them to be identical to Ramicrusta textilis previously reported overgrowing corals in Jamaica. Specimens of PAC sampled from the Honduras were likewise identical, confirming that this crustose alga inhabits the easternmost and westernmost regions of the Caribbean. We also analysed 16S rDNA tag amplicon libraries of the biofilms associated with PAC and sympatric CCA, which is favoured for coral settlement. Our results show that the microbial communities on PAC (vs. CCA) are characterized by significantly lower numbers of the epibiotic bacterial genus Pseudoalteromonas, which facilitates the recruitment and settlement of marine invertebrates. From these data, we infer that PAC are therefore unlikely to be attractive as settlement sites for coral larvae. Given the significant ecological change anticipated on these reefs due to increasing cover of PAC, there is an urgent need to further investigate competitive interactions between PAC and scleractinian corals, and elucidate the role of PAC and their associated microbiomes in accentuating phase shifts from coral to algae on tropical reefs.

Contrasting effects of crustose coralline algae from exposed and subcryptic habitats on coral recruits

Coral recruitment is important in sustaining coral reef ecosystems and contributing to their recovery after disturbances. Despite widespread acceptance that crustose coralline algae (CCA) positively influence coral recruitment success, especially by enhancing coral settlement and early post-settlement stages, there are no experimental data on the effects of CCA species on late post-settlement survival and growth of corals. This study tested the impact of four common, thick-crusted CCA species from two habitats (exposed and subcryptic) on the survival and growth of two recruit size categories of the coral genus Pocillopora. Coral recruits and CCA were transplanted adjacent to each other using epoxy in Petri dishes directly attached to the reef substratum on the forereef of Moorea (French Polynesia) in a 1-year field experiment. In the subcryptic habitat, survival of small-sized recruits adjacent to subcryptic CCA (0–5%) was lower than adjacent to dead CCA (35%), while in the exposed habitat, survival of small-sized recruits adjacent to exposed CCA (20–25%) was higher than adjacent to dead CCA (5%). None of the CCA species affected the survival of large-sized recruits within exposed or subcryptic habitats. However, the growth of large-sized recruits adjacent to subcryptic CCA was lower than adjacent to dead CCA. Recruits adjacent to exposed CCA died less from competition with turf algae relative to dead CCA, while recruits adjacent to subcryptic CCA frequently died from overgrowth by CCA. These results suggest that, in subcryptic habitats, CCA can reduce the survival and/or growth of coral recruits via direct competitive overgrowth, while in exposed habitats, they can enhance coral recruit survival by alleviating competition with turf algae. Importantly, our study demonstrates that not all CCA species are beneficial to the survival and growth of coral recruits and that there is considerable variability in both the outcome and process of competition between CCA and corals.

Crustose coralline algae that promote coral larval settlement harbor distinct surface bacterial communities

Most benthic invertebrates, including ecosystem engineers such as corals, sponges and bivalves, have a motile planktonic larval phase and rely on specific chemical cues to identify a suitable substrate to settle. Crustose coralline algae (CCA) can induce settlement and metamorphosis responses in many invertebrates including corals. We tested the respective coral settlement capacity of multiple CCA species in a choice experiment and investigated the composition of their microbiomes. Our findings revealed that coral larval settlement was drastically influenced by CCA genera and also suggest that bacterial communities on the CCA surface can potentially serve as a driver of coral larval settlement. The composition of the bacterial communities on the surface of the least attractive CCA genus, Neogoniolithon fosliei, was markedly different from the other genera, Porolithon gardineri and Titanoderma prototypum and was significantly enriched in Vibrio and Flammeovirgaceae. The activity of CCA-associated bacterial communities may contribute to some of the variability observed in settlement responses between CCA species. Specific bacterial ASVs assigned to the Neptuniibacter, Methylotrophic Group 3 and Cellvibrionaceae were positively correlated with coral settlement. Conversely, ASVs assigned as Vibrio and Flammeovirga were negatively correlated with coral settlement. This study identifies putative bacterial taxa involved in coral settlement, which is an essential step to understand the chemical cues involved in this process and to predict the ability of corals to recolonize damaged reefs following disturbances.

Inorganic carbon uptake strategies in coralline algae: Plasticity across evolutionary lineages under ocean acidification and warming

Dissolved inorganic carbon (DIC) assimilation is essential to the reef-building capacity of crustose coralline algae (CCA). Little is known, however, about the DIC uptake strategies and their potential plasticity under ongoing ocean acidification (OA) and warming. The persistence of CCA lineages throughout historical oscillations of pCO2 and temperature suggests that evolutionary history may play a role in selecting for adaptive traits. We evaluated the effects of pCO2 and temperature on the plasticity of DIC uptake strategies and associated energetic consequences in reef-building CCA from different evolutionary lineages. We simulated past, present, moderate (IPCC RCP 6.0) and high pCO2 (RCP 8.5) and present and high (RCP 8.5) temperature conditions and quantified stable carbon isotope fractionation (13ε), organic carbon content, growth and photochemical efficiency. All investigated CCA species possess CO2-concentrating mechanisms (CCMs) and assimilate CO2 via diffusion to varying degrees. Under OA and warming, CCA either increased or maintained CCM capacity, which was associated with overall neutral effects on metabolic performance. More basal taxa, Sporolithales and Hapalidiales, had greater capacity for diffusive CO2 use than Corallinales. We suggest that CCMs are an adaptation that supports a robust carbon physiology and are likely responsible for the endurance of CCA in historically changing oceans.

Plasticity of adult coralline algae to prolonged increased temperature and pCO2 exposure but reduced survival in their first generation.

Crustose coralline algae (CCA) are vital to coral reefs worldwide, providing structural integrity and inducing the settlement of important invertebrate larvae. CCA are known to be impacted by changes in their environment, both during early development and adulthood. However, long-term studies on either life history stage are lacking in the literature, therefore not allowing time to explore the acclimatory or potential adaptive responses of CCA to future global change scenarios. Here, we exposed a widely distributed, slow growing, species of CCA, Sporolithon cf. durum, to elevated temperature and pCO2 for five months and their first set of offspring (F1) for eleven weeks. Survival, reproductive output, and metabolic rate were measured in adult S. cf. durum, and survival and growth were measured in the F1 generation. Adult S. cf. durum experienced 0% mortality across treatments and reduced their O2 production after five months exposure to global stressors, indicating a possible expression of plasticity. In contrast, the combined stressors of elevated temperature and pCO2 resulted in 50% higher mortality and 61% lower growth on germlings. On the other hand, under the independent elevated pCO2 treatment, germling growth was higher than all other treatments. These results show the robustness and plasticity of S. cf. durum adults, indicating the potential for them to acclimate to increased temperature and pCO2. However, the germlings of this species are highly sensitive to global stressors and this could negatively impact this species in future oceans, and ultimately the structure and stability of coral reefs.

Settlement dynamics and recruitment responses of Mediterranean gorgonians larvae to different crustose coralline algae species

Sessile marine species such as Anthozoans act as ecosystem engineers due to their three-dimensional structure. Gorgonians, in particular, can form dense underwater forests that give shelter to other species increasing local biodiversity. In the last decades, several Mediterranean gorgonian populations have been affected by natural and anthropogenic impacts which drastically reduced their size. However, some species showed unexpected resilience, mainly due to the supply of new individuals. To understand the mechanisms underlying recovery processes, studies on the first life history stages (i.e. larval dispersal, settlement and recruitment) are needed. In tropical coral reefs, crustose coralline algae (CCA) are known to influence coral larvae habitat selection and settlement. This capacity however is not ubiquitous among CCA species and larvae of different coral species may have different preferences. The present work focuses on three Mediterranean gorgonians (Eunicella singularis, Paramuricea clavata and Corallium rubrum) with the objective of quantifying settlement and recruitment in presence of two common CCA species (Litophyllum stictaeforme and Litophyllum incrustans). Results showed that the presence of CCA activates earlier settlement in E. singularis and increases the density of recruits, with different trends for the three species. Our results suggest that CCA should be taken into account in the implementation of conservation strategies. Moreover, a deeper comprehension of settlement mechanisms could help improving restoration techniques based on sexual reproduction.

Species-Specific Differences in the Microbiomes and Organic Exudates of Crustose Coralline Algae Influence Bacterioplankton Communities.

Crustose coralline algae (CCA) are critical members of the coral reef ecosystem, yet they remain poorly studied. Recent research on CCA has shown that only a few species play a significant role in the settlement of coral larvae through either the production of chemical settlement cues or the facilitation of specific microbial communities that are hypothesized to influence coral settlement. Thus, defining how DOM exudates differ between CCA species and the bacterioplankton communities these exudates facilitate is important for understanding the role of CCA in invertebrate settlement. We conducted single day exudation experiments on two species of CCA to compare tissue microbiome community structure, DOM production and the effect of DOM on the bacterioplankton community. We collected exudates from Hydrolithon reinboldii and Porolithon onkodes in both filter-sterilized seawater and unfiltered seawater from Kāne‘ohe Bay, Hawai‘i. Our results demonstrate that while both species exude equivalent quantities of dissolved organic carbon they differ in the composition of fluorescent DOM and fostered distinct microbial communities. P. onkodes exudates facilitate more microbial OTUs associated with coral disease, whereas H. reinboldii facilitated OTUs known to produce antimicrobial compounds. Our results highlight species-specific differences in the composition of fDOM exudates of CCA and the effect of those on microbial community structure.

De novo transcriptome assembly for four species of crustose coralline algae and analysis of unique orthologous genes

Crustose coralline algae (CCA) are calcifying red macroalgae that reef build in their own right and perform essential ecosystem functions on coral reefs worldwide. Despite their importance, limited genetic information exists for this algal group. De novo transcriptomes were compiled for four species of common tropical CCA using RNA-seq. Sequencing generated between 66 and 87 million raw reads. Transcriptomes were assembled, redundant contigs removed, and remaining contigs were annotated using Trinotate. Protein orthology analysis was conducted between CCA species and two noncalcifying red algae species from NCBI that have published genomes and transcriptomes, and 978 orthologous protein groups were found to be uniquely shared amongst CCA. Functional enrichment analysis of these ‘CCA-specific’ proteins showed a higher than expected number of sequences from categories relating to regulation of biological and cellular processes, such as actin related proteins, heat shock proteins, and adhesion proteins. Some proteins found within these enriched categories, i.e. actin and GH18, have been implicated in calcification in other taxa, and are thus candidates for involvement in CCA calcification. This study provides the first comprehensive investigation of gene content in these species, offering insights not only into the evolution of coralline algae but also of the Rhodophyta more broadly.

Colonization, Growth and Productivity of Crustose Coralline Algae in Sunlit Reefs in the Atlantic Southernmost Coral Reef

One of the most important contributions of crustose coralline algae (CCA) to some coral reefs is their structural role in sunlit habitats, but in the Atlantic southernmost coral reef, Abrolhos, these algae are also important components of living communities covering larger areas than corals. Little is known about their competence in occupying reef space and consequently their ecological role. This work compared two CCA species along reef sites and habitats and their responses to different irradiance levels. To study colonization, epoxy discs were placed at four sites and three habitats (reef base, reef flat and reef edge). Crustose coralline individual pieces were glued onto epoxy discs and their relative growth was estimated. Productivity responses to irradiance levels found on reef habitats was measured on incubated samples. In general, CCA were less abundant than filamentous algae and non-calcareous crusts. Crustose algae showed no seasonal or spatial pattern in cover, contrasting with erect algae that differed in biomass among sites depending on season. Differences among habitats were only found for CCA. The dominant coralline Porolithon onkodes was more productive and grew faster than Lithophyllum stictaeforme at high irradiance level and both species were inhibited at low light. Dominance of P. onkodes in shallow and sunlit reefs was explained by its preference for high-light environments.

Seasonal growth and calcification of a reef-building crustose coralline alga on the Great Barrier Reef

Crustose coralline algae (CCA) play a central role in tropical reef ecology, helping to build and stabilise the reef framework and, due to their high Mg-calcite skeleton, are potential indicators for assessing the impacts of ocean acidification. However, basic information on CCA seasonal growth and calcification is relatively unknown on the Great Barrier Reef (GBR). This study provides seasonal baseline rates of vertical growth, marginal (horizontal) growth and calcification of the abundant CCA species Porolithon onkodes, together with the effect of reduced irradiance on these metrics to better understand the natural variability between CCA living in exposed and shaded areas. Seasonal variation was detected in each of the metrics, with maximum vertical growth and calcification observed in spring and marginal extension in autumn. Annual vertical growth rate was 1.45 mm yr(-1), absolute marginal growth rate (surface area) was 11.12 cm(2) yr(-1), and absolute marginal extension rate (diameter) was 24.66 mm yr(-1). Net calcification, determined using the buoyant weight method (includes secondary calcification deposits) was approximately 2.72 to 3.40 g CaCO3 cm(-2) yr(-1), while gross calcification determined using growth metrics (just newly deposited CaCO3) was 0.43 to 0.59 g CaCO3 cm(-2) yr(-1). Shaded treatments yielded higher growth and gross calcification rates compared to exposed in all seasons except summer. This data provides empirical information necessary to monitor the impacts of future climate change on the GBR and to better understand the response of CCA to manipulative experiments on ocean acidification and warming.